This invention relates to a Tb-activated gadolinium oxysulfide (Gd.sub.1-x Tb.sub.x).sub.2 O.sub.2 S) having improved emission in the blue region of the color spectrum, and acceptable decay characteristics for X-ray intensifier screen applications.
Tb-activated gadolinium oxysulfide (hereinafter sometimes referred to as "GOS:Tb" or "GOST") has been known to be useful as an X-ray-excited phosphor since at least the early 1970's, and as a cathode ray-excited phosphor prior to that time.
In "Evaulation of Gd.sub.2 O.sub.2 S:Tb As A Phosphor For The Input Screen Of X-Ray Image Intensifier," published in IEEE Transactions on Nuclear Science, 19(4), 3-8 (1972), Ludwig et al. compared screens of a GOS:Tb phosphor, containing 0.0008 mole fraction Tb, with conventional (ZnCd)S:Ag screens, and found the GOS:Tb screens to be brighter.
U.S. Pat. No. 3,725,704, issued to Buchanan et al. on Apr. 3, 1973, discloses an X-ray conversion screen including a GOS-Tb phosphor, wherein Tb concentration ranges from about 0.005 up to 8 percent of the Gd concentration. The patent teaches that the optimum concentration for Tb ranges from 0.03 to 3 percent, presumably based upon X-ray-to-light conversion efficiency as plotted in FIG. 3 of the patent.
In U.S. Pat. No. 3,705,858, issued to Luckey et al. on Dec. 12, 1972, there is disclosed a method for preparing lanthanide oxysulfides including GOS:Tb, wherein Tb is present in a mole fraction range of about 0.0001 to 0.10. The method is said to produce high efficiency, low lag phosphors useful, for example, in X-ray screens.
In "X-Ray Exposure Reduction Using Rare-Earth Oxysulfide Intensifying Screens", presented at a 1971 meeting of the Radiological Society of North America, and published in Radiology, 105(1), 185-90 (1972), Buchanan et al. pointed out that the blue emission of GOS:Tb could be enhanced by reducing Tb below its otherwise optimum value. Unfortunately, further investigations have shown that low Tb activator levels are often inexplicably accompanied by undesirable increases in persistence. (Persistence is a measure of the tendency for a phosphor to continue to emit radiation after excitation has ceased).